Precious metals are used as protective films on surfaces for a variety of reasons. In the jewelry trade, it is used to improve the appearance of an article as in gold plated jewelry. In other applications, it is used to protect against corrosion of metals and other surface materials. In the electrical arts protective films made of precious metals are used as conduction paths in electrical circuits and as contact surfaces in devices with electrical contacts. Gold is used extensively in these applications with great success. However, the increased price of gold makes it attractive to look at other precious metals as protective films on various surfaces.
Palladium and palladium alloys are used extensively in a variety of industrial applications. Typical examples are the jewelry trade where such films are used to protect surfaces against corrosion and to improve appearance, in the electrical arts in various electrical devices and electronic circuits and in the optical field for various types of optical devices.
Because of chemical inertness and reasonable hardness, palladium is especially attractive as an electrical contact material in electrical connectors, relay contacts, switches, etc. Various palladium alloys such as palladium-silver, palladium-nickel and palladium-copper are also useful for the same applications. Indeed, because of the increasing cost of gold, palladium and palladium alloys become more and more attractive economically as a contact material, surface material, and in other applications where gold is traditionally used.
A particularly difficult problem in the commercial electroplating of palladium, especially at high plating rates, is accumulation of ions in solution from replenishment of palladium in the bath and maintenance of hydroxide concentration of the bath. Actually, in most situations, these problems are related in that a base (i.e., KOH) is often added to the bath to neutralize hydrogen ions liberated in the electroplating process. The addition of base leads to the accumulation of cations (potassium in the case of KOH). Also, the replenishment of palladium leads to accumulation of anions (i.e., chlorine ions where palladium chloride is used to replenish the bath). Such accumulation limits the lifetime of the bath, usually because an important ingredient precipitates out of the electroplating bath (typically the buffer such as, K.sub.2 HPO.sub.4). Further, accumulation of ions varies the chemical and physical properties of the bath as a function of time. Such a change might vary the properties of the electroplated palladium as a function of time or vary the amount of palladium metal electroplated per unit of electricity.
Most important from a commercial point of view, it limits the lifetime of the electroplating bath, necessitating more frequent bath changes. This is both troublesome, requiring interruptions in the plating process, and leads to waste of valuable chemicals. In addition, uncertainty and variation in the amount of palladium electroplated per unit of plating current requires that excessive palladium metal be electroplated to ensure a minimum required thickness. Such a procedure is wasteful of palladium metal.
It is highly desirable to use a replenishment scheme which neutralizes accumulated acid produced in the electroplating process and does not lead to accumulation of anions. Such a compound is palladium oxide (or the hydrated form Pd(OH).sub.2) but this compound has limited solubility in the palladium electroplating bath and dissolves only very slowly. It is highly desirable to have a preparation procedure for palladium hydroxide in which the palladium hydroxide remains or is formed in a state that is highly and rapidly soluble in the palladium electroplating bath. In addition, palladium recovered from scrap such as electrical contact surfaces is usually recovered in the form of Pd(NH.sub.3).sub.2 Cl.sub.2 and a procedure for preparing palladium hydroxide from this compound is highly desirable.